One of the most striking results of the mid-1990s was the realisation that all of the superstring theories are in fact dual to one another at strong coupling. This also brought eleven dimensional supergravity into the picture and started the search for M-theory, the dynamical theory within which all of those theories would fit as various effective descriptions of perturbative limits. All of this is referred to as the ‘Second Superstring Revolution’. Every revolution is supposed to have a hero or heroes. We shall consider branes to be cast in that particular role, since they (and D-branes especially) supplied the truly damning evidence of the strong coupling fate of the various string theories.

We shall discuss aspects of this in the present section. We simply study the properties of D-branes in the various string theories, and then trust to that fact that as they are BPS states, many of these properties will survive at strong coupling.

Type IIB/type IIB duality

D1-brane collective coordinates

Let us first study the D1-brane. This will be appropriate to the study of type IIB and the type I string by Ω-projection. Its collective dynamics as a BPS soliton moving in flat ten dimensions is captured by the 1+1 dimensional world-volume theory, with 16 or 8 supercharges, depending upon the theory we are in. (See figure 12.1.(a).)